// Copyright 2011 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef V8_DOUBLE_H_
#define V8_DOUBLE_H_

#include "src/base/macros.h"
#include "src/diy-fp.h"

namespace v8 {
namespace internal {

    // We assume that doubles and uint64_t have the same endianness.
    inline uint64_t double_to_uint64(double d) { return bit_cast<uint64_t>(d); }
    inline double uint64_to_double(uint64_t d64) { return bit_cast<double>(d64); }

    // Helper functions for doubles.
    class Double {
    public:
        static constexpr uint64_t kSignMask = V8_2PART_UINT64_C(0x80000000, 00000000);
        static constexpr uint64_t kExponentMask = V8_2PART_UINT64_C(0x7FF00000, 00000000);
        static constexpr uint64_t kSignificandMask = V8_2PART_UINT64_C(0x000FFFFF, FFFFFFFF);
        static constexpr uint64_t kHiddenBit = V8_2PART_UINT64_C(0x00100000, 00000000);
        static constexpr int kPhysicalSignificandSize = 52; // Excludes the hidden bit.
        static constexpr int kSignificandSize = 53;

        Double()
            : d64_(0)
        {
        }
        explicit Double(double d)
            : d64_(double_to_uint64(d))
        {
        }
        explicit Double(uint64_t d64)
            : d64_(d64)
        {
        }
        explicit Double(DiyFp diy_fp)
            : d64_(DiyFpToUint64(diy_fp))
        {
        }

        // The value encoded by this Double must be greater or equal to +0.0.
        // It must not be special (infinity, or NaN).
        DiyFp AsDiyFp() const
        {
            DCHECK_GT(Sign(), 0);
            DCHECK(!IsSpecial());
            return DiyFp(Significand(), Exponent());
        }

        // The value encoded by this Double must be strictly greater than 0.
        DiyFp AsNormalizedDiyFp() const
        {
            DCHECK_GT(value(), 0.0);
            uint64_t f = Significand();
            int e = Exponent();

            // The current double could be a denormal.
            while ((f & kHiddenBit) == 0) {
                f <<= 1;
                e--;
            }
            // Do the final shifts in one go.
            f <<= DiyFp::kSignificandSize - kSignificandSize;
            e -= DiyFp::kSignificandSize - kSignificandSize;
            return DiyFp(f, e);
        }

        // Returns the double's bit as uint64.
        uint64_t AsUint64() const
        {
            return d64_;
        }

        // Returns the next greater double. Returns +infinity on input +infinity.
        double NextDouble() const
        {
            if (d64_ == kInfinity)
                return Double(kInfinity).value();
            if (Sign() < 0 && Significand() == 0) {
                // -0.0
                return 0.0;
            }
            if (Sign() < 0) {
                return Double(d64_ - 1).value();
            } else {
                return Double(d64_ + 1).value();
            }
        }

        int Exponent() const
        {
            if (IsDenormal())
                return kDenormalExponent;

            uint64_t d64 = AsUint64();
            int biased_e = static_cast<int>((d64 & kExponentMask) >> kPhysicalSignificandSize);
            return biased_e - kExponentBias;
        }

        uint64_t Significand() const
        {
            uint64_t d64 = AsUint64();
            uint64_t significand = d64 & kSignificandMask;
            if (!IsDenormal()) {
                return significand + kHiddenBit;
            } else {
                return significand;
            }
        }

        // Returns true if the double is a denormal.
        bool IsDenormal() const
        {
            uint64_t d64 = AsUint64();
            return (d64 & kExponentMask) == 0;
        }

        // We consider denormals not to be special.
        // Hence only Infinity and NaN are special.
        bool IsSpecial() const
        {
            uint64_t d64 = AsUint64();
            return (d64 & kExponentMask) == kExponentMask;
        }

        bool IsInfinite() const
        {
            uint64_t d64 = AsUint64();
            return ((d64 & kExponentMask) == kExponentMask) && ((d64 & kSignificandMask) == 0);
        }

        int Sign() const
        {
            uint64_t d64 = AsUint64();
            return (d64 & kSignMask) == 0 ? 1 : -1;
        }

        // Precondition: the value encoded by this Double must be greater or equal
        // than +0.0.
        DiyFp UpperBoundary() const
        {
            DCHECK_GT(Sign(), 0);
            return DiyFp(Significand() * 2 + 1, Exponent() - 1);
        }

        // Returns the two boundaries of this.
        // The bigger boundary (m_plus) is normalized. The lower boundary has the same
        // exponent as m_plus.
        // Precondition: the value encoded by this Double must be greater than 0.
        void NormalizedBoundaries(DiyFp* out_m_minus, DiyFp* out_m_plus) const
        {
            DCHECK_GT(value(), 0.0);
            DiyFp v = this->AsDiyFp();
            bool significand_is_zero = (v.f() == kHiddenBit);
            DiyFp m_plus = DiyFp::Normalize(DiyFp((v.f() << 1) + 1, v.e() - 1));
            DiyFp m_minus;
            if (significand_is_zero && v.e() != kDenormalExponent) {
                // The boundary is closer. Think of v = 1000e10 and v- = 9999e9.
                // Then the boundary (== (v - v-)/2) is not just at a distance of 1e9 but
                // at a distance of 1e8.
                // The only exception is for the smallest normal: the largest denormal is
                // at the same distance as its successor.
                // Note: denormals have the same exponent as the smallest normals.
                m_minus = DiyFp((v.f() << 2) - 1, v.e() - 2);
            } else {
                m_minus = DiyFp((v.f() << 1) - 1, v.e() - 1);
            }
            m_minus.set_f(m_minus.f() << (m_minus.e() - m_plus.e()));
            m_minus.set_e(m_plus.e());
            *out_m_plus = m_plus;
            *out_m_minus = m_minus;
        }

        double value() const { return uint64_to_double(d64_); }

        // Returns the significand size for a given order of magnitude.
        // If v = f*2^e with 2^p-1 <= f <= 2^p then p+e is v's order of magnitude.
        // This function returns the number of significant binary digits v will have
        // once its encoded into a double. In almost all cases this is equal to
        // kSignificandSize. The only exception are denormals. They start with leading
        // zeroes and their effective significand-size is hence smaller.
        static int SignificandSizeForOrderOfMagnitude(int order)
        {
            if (order >= (kDenormalExponent + kSignificandSize)) {
                return kSignificandSize;
            }
            if (order <= kDenormalExponent)
                return 0;
            return order - kDenormalExponent;
        }

    private:
        static constexpr int kExponentBias = 0x3FF + kPhysicalSignificandSize;
        static constexpr int kDenormalExponent = -kExponentBias + 1;
        static constexpr int kMaxExponent = 0x7FF - kExponentBias;
        static constexpr uint64_t kInfinity = V8_2PART_UINT64_C(0x7FF00000, 00000000);

        // The field d64_ is not marked as const to permit the usage of the copy
        // constructor.
        uint64_t d64_;

        static uint64_t DiyFpToUint64(DiyFp diy_fp)
        {
            uint64_t significand = diy_fp.f();
            int exponent = diy_fp.e();
            while (significand > kHiddenBit + kSignificandMask) {
                significand >>= 1;
                exponent++;
            }
            if (exponent >= kMaxExponent) {
                return kInfinity;
            }
            if (exponent < kDenormalExponent) {
                return 0;
            }
            while (exponent > kDenormalExponent && (significand & kHiddenBit) == 0) {
                significand <<= 1;
                exponent--;
            }
            uint64_t biased_exponent;
            if (exponent == kDenormalExponent && (significand & kHiddenBit) == 0) {
                biased_exponent = 0;
            } else {
                biased_exponent = static_cast<uint64_t>(exponent + kExponentBias);
            }
            return (significand & kSignificandMask) | (biased_exponent << kPhysicalSignificandSize);
        }
    };

} // namespace internal
} // namespace v8

#endif // V8_DOUBLE_H_
